1. Field of the Invention
The present invention relates to a low-temperature process for fabricating an insulated gate semiconductor device at a temperature as low as 450.degree. C. or even lower, and to a process for fabricating, at good yield, an integrated circuit (IC) comprising said devices at a high degree of integration. The present invention also relates to a semiconductor device having fabricated by the above process. It further relates to a highly reliable semiconductor device. The semiconductor devices according to the present invention are suited for use in, for example, active matrix-driven liquid crystal displays, driver circuits of image sensors, etc., as well as thin film transistors for SOI integrated circuits and for conventional semiconductor integrated circuits (e.g., microprocessors, microcontrollers, microcomputers, semiconductor memories, etc.).
2. Prior Art
Recently, much effort is paid on the study of fabricating insulated gate semiconductor devices on insulator substrates (MOSFETs). Such devices comprising semiconductor integrated circuits on insulator substrates are advantageous for driving circuits at high speed. In contrast to the conventional semiconductor ICs whose speed is limited by the presence of a stray capacitance attributed principally to the capacitance between the connection and the substrate, the new type of semiconductor integrated circuits do not suffer such stray capacitance. The above MOSFET having a thin film active layer on an insulator substrate is denoted as a thin film transistor (TFT). The TFT can be found also in the conventional semiconductor ICs as, for example, a load transistor for SRAMs.
More recently, there is a demand for fabricating semiconductor ICs on a light-transmitting substrate, for example, as driver circuits in optical devices such as liquid crystal displays and image sensors. TFTs are also useful in such application fields. The circuits for use therein should, however, be formed over a large area. The process is therefore required to be conducted at a ever lower temperature. Furthermore, for example, when there is a need of connecting a semiconductor IC to the terminals of a device having a plurality of terminals on an insulator substrate, it is proposed to form monolithically the entire semiconductor IC or to form at least the initial stages thereof monolithically on the same insulator substrate.
Conventionally, TFTs have been fabricated by annealing an amorphous, a semi-amorphous, or a microcrystalline semiconductor film in the temperature range of from 450 to 1,200.degree. C. to obtain a crystalline film having an improved crystallinity and having a sufficiently high mobility. TFTs include amorphous TFTs using an amorphous material as the semiconductor film, however, such TFTs are not useful as they are because they yield a mobility as low as 5 cm.sup.2 /Vs or even lower, and in general, the mobility falls to a value of about 1 cm.sup.2 /Vs or lower. The use of amorphous TFTs as they are is confined to a narrow range of application because of its low operation speed and its limited applicability to N-channel type TFTs. Accordingly, these TFTs were annealed in the aforementioned temperature range to attain a mobility of 5 cm.sup.2 /Vs or higher. Only after annealing, these TFTs can provide P-channel TFTs (PTFTs).
A thermal process as described in the foregoing has, however, strict limitations on the material to be used as the substrate. In a so-called high temperature process which comprises a step of heating to a temperature in the range of from 900 to 1,200.degree. C. at maximum, a thermally oxidized film of superior quality can be used as the gate dielectric. Thus, expensive substrates such as those made of quartz and sapphire and spinel were the only candidates applicable to such high temperature processes. Moreover, large area substrates were rarely obtained with such expensive materials.
In contrast to the case of a high-temperature process, variety of substrate materials can be selected for use in a low temperature process which is conducted at temperatures which do not exceed the range of from 450 to 750.degree. C. However, a low temperature process requires annealing for a long time, and the substrates resulting therefrom suffer strain and shrinking due to the heat effect.
Furthermore, it is extremely difficult in a MISFET, i.e., an insulated gate semiconductor device having formed on an insulated surface established by incorporating a thick insulator film between a semiconductor substrate and the device to isolate the device from the semiconductor substrate, to obtain an element having favorable crystallinity as in the case using a single crystal semiconductor. Accordingly, a non-single crystalline semiconductor, i.e., a crystalline semiconductor other than a single crystal semiconductor, had been used generally in MISFETS.
The non-single crystalline semiconductors comprise defects at high density, and are usually neutralized previously with an element such as hydrogen to use them in a practically defect-free state. The neutralization process can be carried out by, for example, hydrogenation. The bond between hydrogen and the semiconductor element such as silicon is generally weak, and would easily undergo breakage to cause decomposition of the resulting compound on applying a thermal energy corresponding to a mere several tens of degrees Centigrade. Accordingly, when electric voltage or current is applied for a long duration of time, hydrogen readily undergoes desorption due to the local heat up of the semiconductor. This phenomena remarkably causes degradation of the semiconductor.
The present invention has been achieved in the light of the aforementioned circumstances. An object of the present invention is, therefore, to provide a process which can be conducted at a temperature not higher than 450.degree. C., which suffer no limitations on the substrate material, and which has no problems of strain and shrinkage. Another object of the present invention is to provide a semiconductor device having such a structure that the heat generated during its usage can be rapidly released, and also to a process for fabricating the device.